GIMP 2.8 promises to bring a new single window interface to make Photoshop users feel more at home. GIMP 2.8 has not been released and it will be quite sometime before it is released. However, if you cannot wait to lay your hands on the single window interface, you can install the latest GIMP 2.7.x from the PPA - ppa:matthaeus123/mrw-gimp-svn. It is actually an unstable version and if you need GIMP to work, do not install it. Otherwise go ahead, but keep in mind that this is not a stable release.

Before we start, is a screenshot:

As you can the new interface also has tabs through which you can browse through the different open images.

Now, to install GIMP 2.7.3, here is what you have to do:

Open the terminal.

Add the PPA with the command:

sudo add-apt-repository ppa:matthaeus123/mrw-gimp-svn

Update software list with:

sudo apt-get update

Finally install with the command

sudo apt-get install gimp

After installation you will get the multi-window interface by default. You can switch to the single window interface with Windows -> Single-window mode.